Allow gerrit-verifiers to edit topic on changes

Currently only change and project owners may edit a topic.

Widen the permission to include gerrit-verifiers, which is
a manually curated group of known and trusted contributors.

Force permission (i.e. changing the topic on merged changes)
is omitted and thus limited to project owners.

Change-Id: I47fa2a3ae02e9535478111a0bfba26d4388b98a2